The invention relates to a method for recovering a bit stream from a radio signal received in a first receiving station and in at least one further receiving station, to a receiving station for recovering a bit stream from a radio signal received in the receiving station and at least one further receiving station, to a base station router comprising at least one such receiving station and a channel element, and to a network for recovering a bit stream from a radio signal.
When a radio signal emitted by a mobile station is received in two or more different receiving stations, it is possible to combine the received signals in such a way that the bit stream which has been emitted by the mobile station can be recovered with a higher quality as compared to the case that the reconstruction is performed based on a single received signal. For recovering such a bit stream by combining two or more received signals, two schemes known as softer handover and soft handover are known in the state of the art, which will be briefly explained in the following with respect to FIGS. 1 and 2.
FIG. 1 shows an example of a hierarchical wireless network 1 with a Radio Network Controller RNC as a central station which is connected to three base stations BS1 to BS3 located at different sites within the network 1. Each of the base stations BS1 to BS3 comprises three sectors, also referred to as receiving stations S1 to S3 in the following, for receiving radio signals which have been emitted by a mobile station MS, e.g. a mobile phone, PDA etc. In the example of FIG. 1, the radio transmission between the base stations BS1 to BS3 and the mobile station MS is implemented using the UMTS (universal mobile telecommunications system) standard, based on a 16-QAM (quadrature amplitude modulation) modulation scheme, using an in-phase (I) and a quadrature-phase (Q) component for encoding the bit stream. For this reason, radio signals transmitted in the network are also referred to as IQ baseband signals in the following.
Radio signals emitted by the mobile station MS are received by each of the three base stations BS1 to BS3. In the first base station BS1, a first and second receiving station S1, S2 receive respective radio signals 2a, 2b from the mobile station MS, whereas in the second and third base stations BS2, BS3, only the second and first receiving stations S2, S1, respectively, are capable of receiving radio signals 3, 4 from the mobile station MS.
For recovering a bit stream from the radio signals 2a, 2b received by the first and second receiving stations S1, S2 of the first base station BS1, these signals are combined in a process which is referred to as softer handover (HO). In softer handover, the radio signals 2a, 2b of the two receiving stations S1, S2 (including their diversity branches) are combined using Maximum Ratio Combining (MRC) which is known as the optimum linear combining scheme of signals with noise, typically producing an output signal OS1 of the first base station BS1 with improved quality as compared to bit streams of the output signals OS2, OS3 from the second and third base station BS2, BS3.
However, for applying the softer handover scheme, a lot of data needs to be routed between the receiving stations S1 to S3 of the first base station BS1, being a base station according to the CPRI/OBSAI (Common Packet Radio Interface/Open Base Station Architecture Initiative) standard shown in greater detail in FIG. 2, each of the three receiving stations S1 to S3 of which are connected to a respective radio frequency (RF) head RF1 to RF3. Of course, two or more radio heads may also be used for each of the receiving stations S1 to S3 when diversity receiving is performed.
The three receiving stations S1 to S3 are connected to a specialized switch fabric 5 to which the received radio signals 2a, 2b are transported according to the CPRI protocol standard, using a huge amount of IQ baseband data, thus requiring a backplane with huge data transport capacity. This is especially problematic if the receiving stations S1 to S3 and their respective RF heads RF1 to RF3 are installed remotely and the CPRI data has to be transported over larger distances in the context of a so-called base station hotel concept. Furthermore, the switch fabric 5 has to support tremendous data rates.
The concept described above is furthermore not easily scalable if more and more receiving stations are to be added with upgrades, as the switch fabric has to be adapted in this case. The same problem occurs with a central channel element 6 after the switch fabric 5 which transforms the CPRI data to IP data at its output. The size of the channel element 6 has to match the final number of receiving stations, such that a “pay as you grow” strategy is not supported with the CPRI/OBSAI approach.
In summary, the backplane of a base station has to offer a tremendous bandwidth in order to support softer handover. As any combination of received signals from multiple receiving stations is possible and as base stations with three sectors and diversity receiving (e.g. using two RF heads per receiving station) are typical, the bandwidth equals six times the bandwidth of the basic IQ baseband data signal. For example, the IQ baseband data may be sampled as 2×14 bit at two samples per chip for UMTS which equals 2×14×2×3.84 MChip/s=215 Mbit/s. Inside the radio cards, an adaptive scaling is performed reducing the resolution from 14 to 5 bit, leading to 76.8 Mbit/s. Considering the six antenna paths arising from three receiving stations plus receiving diversity this implies a total bandwidth of 76.8×6=460.1 Mbit/s, requiring an extremely costly backplane.
For combining the output signals OS1 to OS3 of the three base stations BS1 to BS3, another scheme called soft handover is used which applies a “frame selection” in the radio network controller RNC, to which the output signals OS1 to OS3 are routed as IP data via respective links L1 to L3.
The frame selection is based on decoded frames of the output signals OS1 to OS3 from the base stations BS1 to BS3. During the frame selection, valid frames are selected from the frames in the output signals OS1 to OS3 by evaluating the CRC (cyclic redundancy check) checksums, in particular the indicator bit transported with each frame, indicating a good or bad frame. Frame selection is not able to truly combine the information of the different output signals OS1 to OS3. Thus, soft handover is a kind of hard decision and may be seen as some sort of selection combining inside the radio network controller RNC, which serves as a concentrator function.
The soft handover scheme used today catches a certain amount of combining gain, but not all. It would be desirable to have also softer handover combining between different base stations, but due to enormous bandwidth requirements in the backhaul, this is not feasible. Using softer handover between the base stations would require to send IQ baseband data received in multiple receiving stations towards the radio network controller RNC. It would also imply that the concentrator function, which today is a simple frame selection task, would become very computationally intense, especially in light that a radio network controller RNC has to handle a few hundred base stations. When performing softer handover, the radio network controller RNC would have to perform digital signal processing tasks and not a simple selection and switching task as today. The radio network controller RNC would also have to be equipped with high performance signal processing DSPs and ASICs for this purpose.
Moreover, in the context of a flat IP network structure, where all network elements are collapsed into Base Station Routers (BSR), there is no longer a central station such as a radio network controller. However, also in a fiat IP network, it is still desirable to offer soft or softer handover.